Structure/method to fabricate a high performance magnetic tunneling junction MRAM

ABSTRACT

An MTJ (magnetic tunneling junction) MRAM (magnetic random access memory) cell is formed on a conducting lead and magnetic keeper layer that is capped by a sputter-etched Ta layer. The Ta layer has a smooth surface as a result of the sputter-etching and that smooth surface promotes the subsequent formation of a lower electrode (pinning/pinned layer) with smooth, flat layers and a radical oxidized (ROX) Al tunneling barrier layer which is ultra-thin, smooth, and to has a high breakdown voltage. A seed layer of NiCr is formed on the sputter-etched layer of Ta. The resulting device has generally improved performance characteristics in terms of its switching characteristics, GMR ratio and junction resistance.

RELATED PATENT APPLICATION

This application is related to Ser. No. 10/371,841, filing date Feb. 20,2003 and Ser. No. 10/768,917, filing date Jan. 30, 2004 and Ser. No.10/849,310 filing date May 19, 2004 assigned to the same assignee as thecurrent invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to magnetic tunneling junction (MTJ)MRAMs and more particularly to the use of a simple fabrication processthat leads to a smooth bottom electrode and superior performanceproperties.

2. Description of the Related Art

The magnetic tunneling junction device (MTJ device) is essentially avariable resistor in which the relative orientation of magnetic fieldsin an upper and lower magnetized electrode controls the flow ofspin-polarized tunneling electrons through a very thin dielectric layer(the tunneling barrier layer) formed between those electrodes. Aselectrons pass through the lower electrode they are spin polarized byits magnetization direction. The probability of an electron tunnelingthrough the intervening tunneling barrier layer then depends on themagnetization direction of the upper electrode. Because the tunnelingprobability is spin dependent, the current depends upon the relativeorientation of the magnetizations of magnetic layers above and below thebarrier layer. Most advantageously, one of the two magnetic layers (thepinned layer) in the MTJ has its magnetization fixed in direction, whilethe other layer (the free layer) has its magnetization free to move inresponse to an external stimulus. If the magnetization of the free layeris allowed to move continuously, as when it is acted on by acontinuously varying external magnetic field, the device acts as avariable resistor and it can be used as a read-head. If themagnetization of the free layer is restricted to only two orientationsrelative to the fixed layer (parallel and anti-parallel), the first ofwhich produces a low resistance (high tunneling probability) and thesecond of which produces a high resistance (low tunneling probability),then the device behaves as a switch, and it can be used for data storageand retrieval (a MRAM).

Magnetic tunneling junction devices are now being utilized asinformation storage elements in magnetic random access memories (MRAMs).Typically, when used as an information storage or memory device, awriting current orients the magnetization of the free layer so that itis either parallel (low resistance) or anti-parallel (high resistance)to the pinned layer. The low resistance state can be associated with abinary 0 and the high resistance state with a binary 1. At a later timea sensing current passed through the MTJ indicates if it is in a high orlow resistance state, which is an indication of whether itsmagnetizations are, respectively, antiparallel or parallel and whetherit is in a 0 or 1 state. Typically, switching the magnetizationdirection of the free layer from parallel to antiparallel and vice-versais accomplished by supply currents to orthogonal conductor lines, onewhich is above the MRAM cell and one which is below it. The line belowthe cell is referred to as the word line and it is electrically isolatedfrom the cell. The line above the cell, called the bit line, is indirect electrical contact with the cell and is used for both writing onthe cell, ie changing the direction of the free layer magnetization andreading the cell, ie detecting the free layer magnetization direction.The two lines pass each other orthogonally, in separated verticalplanes, with the cell lying between them. Thus their combined fieldpeaks just above the switching threshold of the cell, the field requiredto cause a transition from parallel to antiparallel orientations of thefree layer and pinned layer magnetizations.

For fast operation, the cell must have a high magnetoresistance ratio(DR/R), where DR represents the resistance variation when the free layerswitches its magnetization direction and R represents the total minimumresistance of the cell. For stable operation, the cell's junctionresistance, RA, where A is cell cross-sectional area, must be wellcontrolled. When the MRAM device is used as the basic element of amemory, it is replicated to form an array of many such devices andintegrated with associated CMOS circuitry which accesses particularelements for data storage and retrieval.

When fabricating an MRAM element or an array of such elements, thenecessity of creating a high value of DR/R and maintaining a high degreeof control over the junction resistance requires the formation of thin,smooth layers of high quality.

Slaughter et al. (U.S. Pat. No. 6,544,801 B1) teaches a method offabricating such a magnetic tunneling device wherein the problem ofinterdiffusion between layers of different metals during hightemperature annealing processes is significantly reduced. Suchinterdiffusion would adversely affect the properties of the variouslayers because of the tendency of the various metals to alloy with eachother.

Dill et al. (U.S. Pat. No. 6,114,719) teaches a method of effectivelybiasing an MTJ device using biasing layers disposed within the devicestack, so that its magnetic states are stable, yet there is not requiredthe addition of adjacent magnetic structures which would adverselyaffect the high device density required for an MRAM array.

In a “standard process” MRAM array structure the MTJ stack (lowerelectrode/AlOx tunneling barrier/upper electrode) is deposited on top ofthe bottom conductor, which is typically a tri-layer such as Ta/Cu/Ta orNiCr/Ru/Ta. In the latter tri-layer, the Ta that caps the Ru is grownwith an α-phase structure, in the former tri-layer, the Ta that caps theCu is grown with a β-phase structure. Prior to depositing the MTJ stackit is necessary to sputter-etch the TaO which grows on the Ta cappinglayer. This sputter-etch not only removes the surface TaO, but theenergetic Ar sputtering ions also alter the Ta surface structure. Theresulting sputter-etched Ta surface appears to be “amorphous-like”,similar to that of amorphous Al₂O₃. In our experiments we have foundthat an altered Ta surface is necessary for forming a flat, smoothbottom electrode on which to most advantageously form an oxidized Altunneling barrier layer of high integrity. It was also noted by us thatrefilling the sputter-etched Ta surface by a Ta sputter-depositionactually results in a rougher surface structure of the bottom electrode.The integrity of the oxidized Al barrier layer is an essential elementin fabricating a high quality MTJ device.

Formation of a high-speed MRAM array is quite complicated. Normally itsword line structure is surrounded by a dielectric layer, so the lineessentially lies within a cavity. This cavity has a back (or bottom)surface and two parallel side surfaces that are spaced apart. The backand/or side surfaces of the cavity are covered with an NiFe magneticlayer which acts as a field keeper (it contains the magnetic flux). Theconducting portion of the line, surrounded by the magnetic keeperstructure, is formed within the cavity. A polishing process is then usedto remove any portion of the keeper or conducting portion of the linethat extends above the level of the dielectric surface and to generallyrender that surface planar.

A novel MRAM array structure, not using the conducting lead structure ofthe standard process, has been developed in which the word line isconstructed on top of the MTJ. The MRAM configuration of this novelarray structure is:

-   NiCr50/NiFe100/NiCr30/Cu50/MnPt100/CoFe18/Ru7.5/CoFe15/Al(8–10)/ROX/free/cap    The NiCr50/NiFe100/NiCr30/Cu50 portion is the bottom conducting lead    (the numerals representing thicknesses in angstroms), which includes    a NiCr 50 seed layer, a100 angstrom NiFe soft adjacent keeper layer,    a second NiCr 30 seed layer and a Cu 50 conducting layer; the    MnPt100/CoFe18/Ru7.5/CoFe15 portion is the bottom electrode (a    synthetic pinned structure), the Al(8–10)/ROX is a tunneling barrier    layer formed by radical oxidation of an 8–10 angstrom thick Al layer    and then there is the upper electrode, which includes a free layer    and a capping layer formed thereon but not described here in detail.    The entire stack is advantageously formed by magnetron sputtering in    a single pump-down of the sputtering chamber.

In initial testing of this single pump-down fabrication, RA (junctionresistance), DR/R and V_(b) (breakdown voltage of the barrier layer)were found to be much lower than values obtained from the standard(prior art) process in which the patterned conductor lead and the MRAMstack are formed separately. Further, high resolution TEM analysis ofthe single pump-down layers showed that the bottom electrode layers hada columnar grain structure, which tended to create rough surfaces. Incontrast, the surfaces of layers formed in the standard process wererelatively smooth and flat. The essential difference in the twoprocesses is that the standard process configuration includes a Tacapping layer that is sputter-etched.

The object of this invention is to modify the single pump-down processso that a smooth, flat layered structure (as in the standard process) isobtained while still maintaining the advantages of the single pump-downof the novel process.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a method of forming anMTJ MRAM element and an array of such elements, that are characterizedby smooth, flat layers in the bottom electrode and an ultra thin, smoothtunneling barrier layer with high breakdown voltage.

A second object of this invention is to provide such an MRAM element andMRAM element array that can be fabricated in a single pump-down process.

The objects of the present invention will be achieved by the fabricationof an MRAM element having the following novel configuration:

-   NiCr50/NiFe100/NiCr30/Cu50/Ta80/Ru30/[SE    Ru30Ta(20–30)]/NiCr40/MnPt100/    CoFe18/Ru7.5/CoFe15/Al(8–10)/ROX/free/cap    Here, an initially deposited Ta80/Ru30 bilayer plays the role of a    capping layer (the Ru protecting the Ta from oxidation) for the    conducting lead layer of Cu50, and then it is sputter-etched (SE) to    remove the Ru 30 and between approximately 20 and 30 angstroms of    the Ta 80, leaving only 50 to 60 angstroms of the Ta prior to    deposition of the NiCr40/MnPt100/CoFe18/Ru7.5/CoFe15 pinned bottom    electrode layer.

MRAM devices manufactured as above have excellent characteristics. Priorto the above configuration, the single pump-down without thesputter-etched Ta capping layer produced values of RA, DR/R and V_(b)that were respectively 0.4–0.8 kΩ-μm², 10–15% and 1.0 volt. Using thenew process, the values obtained are 3.0–4.0 kΩ-μm², 38% and 1.6 volts.These values are comparable to those obtained in the prior multiple stepprocess. In addition to these improved characteristic values, the MRAMdevice of the present invention also has excellent switchingcharacteristics, with MR v. field curves showing simple rectangularshapes with no jumps or kinks in hysteresis loops. For a cell of 0.2×0.4μm² cross-sectional area the switching current required is about 2 ma,compared to a switching current of about 5 ma for the conventionalprocess elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a–e are schematic cross-sectional views of the formation of anMTJ MRAM device on a conducting lead layer using the method andconfiguration of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method of forming an MTJ MRAM with a tunnelbarrier layer of high integrity, in a single pump-down process, by theintroduction of a sputter-etched Ta capping layer formed on the lowerbit line. The sputter-etched Ta layer promotes the subsequent formationof smooth, flat layers within the bottom electrode and thereby allowsthe formation of a thin, smooth and flat tunnel barrier layer made bysubjecting a thin Al layer to a process of radical oxidation (ROX).

Referring first to FIG. 1 a, there is seen in a schematic cross sectionan initial stage of a preferred embodiment of the invention, theformation of a single MRAM element, which can be a part of an array ofsuch elements. It is to be understood that in the embodiments to bedisclosed in what follows, all layer depositions take place in a highvacuum system suitable for depositing thin layers by sputtering. Inthese embodiments the system was a commercially available Anelva 7100system which includes ultra-high vacuum sputtering-deposition chambersand oxidation chambers, but other comparable systems are also suitable.It is also noted that in all the embodiments to be discussed, when thetunneling barrier layer was to be oxidized, the fabrication alreadyformed (having the un-oxidized, as-deposited Al layer on it) was removedfrom the high vacuum system and placed in a separate oxidation chamberfor the oxidation process to occur. Subsequent to the oxidation, thefabrication is replaced into the high vacuum sputtering chamber for theremaining layer depositions.

There is shown first a substrate (10), which in this embodiment is adielectric layer formed on a silicon substrate. A lower conducting lead(the bit line), on which the MRAM element will be formed, is depositedon the substrate. This invention includes the formation of the bit line,which contains a soft magnetic keeper structure, on the substrate andthe subsequent formation of the MRAM stack on the bit line. It isfurther understood that the single MRAM element to be described can beone of an array of such elements and that such element or array may befurther connected to associated circuitry used in storing and retrievinginformation.

On the substrate (10), there is deposited a first seed layer (20), whichin this embodiment is a layer of NiCr(35%–45%) formed to a thicknessbetween approximately 50 and 100 angstroms, with approximately 50angstroms being preferred. The percentages refer to percent of atoms ofCr in the NiCr alloy. On the seed layer is then formed a keeper layer(25) of soft magnetic material, which in this embodiment is a layer ofNiFe formed to a thickness between approximately 50 and 200 angstromswith 100 angstroms being preferred. On the keeper layer there is formeda conducting lead layer (35) which can have two different structures. Inone structure it includes a (third) seed layer (30), which is a layer ofNiCr (40%) formed to a thickness between approximately 20 and 50angstroms and which will serve as a seed layer for the subsequentlydeposited conductor layer. On the seed layer (30) there is then formed alayer of conducting material (40), which in this form is a layer of Cuof thickness between approximately 50 and 100 angstroms with 50angstroms being preferred. In a second structure, there is firstdeposited a layer of Ta (30) of thickness between approximately 50 and100 angstroms with 50 angstroms being preferred. On the Ta layer (30)there is then formed a layer of conducting material (40), which in thisembodiment is a layer of Cu of thickness between approximately 50 and100 angstroms with 50 angstroms being preferred.

On either structure of the conducting layer there is then formed acapping layer (50) of Ta, of thickness between approximately 50 and 100angstroms with 80 angstroms being preferred. A layer of Ru (55), ofthickness between approximately 20 and 40 angstroms with 30 angstromsbeing preferred is then formed on the Ta capping layer to protect itfrom oxidation.

Referring next to FIG. 1 b, there is shown this Ta/Ru bilayer as thenbeing sputter-etched to remove the Ru entirely and to remove betweenapproximately 20 and 30 angstroms of the as-deposited Ta layer, nowdenoted (53). This sputter-etching interrupts the columnar grain growthof the Ta which, if not interrupted, would produce a rough surface andpoor quality subsequent layer structure. Instead, the sputter-etched Tahas a smooth surface which is characteristic of amorphous materiallayers.

Referring now to FIG. 1 c, there is shown the initial steps in theformation of the bottom electrode (the pinned layer) on thesputter-etched Ta capping layer. It is the feature of this inventionthat the layers of this electrode will be flat and smooth as a result ofbeing formed on the sputter-etched Ta capping layer. First, a secondseed layer (70), which in this embodiment is a layer of NiCr(40%) formedto a thickness between approximately 40 and 50 angstroms is formed onthe sputter-etched Ta layer (53). A pinned/pinning layer (80) is thenformed on the NiCr layer. The layer includes an antiferromagneticpinning layer (82), which in this embodiment is a layer of MnPt formedto a thickness between approximately 100 and 200 angstroms, withapproximately 150 angstroms being preferred. It is noted that a thinnerlayer of IrMn can be substituted for the MnPt if a thinner structure isrequired in order to produce a smaller spacing (and larger correspondingmagnetic field) between the keeper layer and the free layer. On thepinning layer there is then formed a synthetic antiferromagnetic pinned(SyAP) layer (84), which in this embodiment is a first ferromagneticlayer (92) of CoFe of thickness between approximately 15 and 25angstroms with 18 angstroms being preferred. On this layer is formed athin coupling layer (94) of Ru, which is formed to a thickness betweenapproximately 7 and 8 angstroms with 7.5 angstroms being preferred. Onthe coupling layer is formed a second ferromagnetic layer (96) of CoFe(25%) with a thickness between approximately 10 and 20 angstroms with 15angstroms being preferred. The 25% by number of atoms of Fe in thislayer of CoFe is found to produce a particularly good value of DR/R.

Still referring to FIG. 1 c, there is shown the first step in theformation of a thin, flat and smooth tunneling barrier layer on thepinned layer in which an Al layer (100) between approximately 7 and 12angstroms thickness with 10 angstroms being preferred is formed on theCoFe(25%) layer (96).

Referring now to FIG. 1 d, there is shown the fabrication of FIG. 1 c,with the as-deposited Al layer (100) thus far formed, removed from thehigh vacuum sputtering-deposition chamber and placed in an oxidationchamber where it is oxidized (shown schematically as arrows) by aprocess of radical oxidation (ROX) in-situ. The oxidized layer is nowdenoted as (110) and other layers and their numeric designation havebeen suppressed for clarity. The details of the oxidation chamber arenot shown. Briefly, the ROX process as applied to achieve the objects ofthe present invention is a plasma oxidation process carried out within aplasma oxidation chamber wherein a grid-like cap is placed between anupper ionizing electrode and the wafer surface being oxidized. Oxygengas is then fed to the upper electrode and power is supplied to theelectrode to at least partially ionize the gas. Passage of the partiallyionized gas through the cap produces a shower of oxygen atoms,molecules, radicals and ions and renders the various species produced bythe electrode less energetic when they arrive at the wafer surface.Within the plasma chamber used herein, an upper electrode within thechamber is fed with 0.5 liters of oxygen gas to produce a shower ofoxygen radicals. Power is supplied to the electrode at a rate of 500 to800 watts. The tunneling barrier layer is thereby formed to exceptionalsmoothness and uniformity and has a high breakdown voltage, all being aresult of its formation over the sputter-etched Ta and NiCr layers.

Referring next to FIG. 1 e, there is shown the formation of free layer(120) on the bottom electrode. The free layer is preferably a layer ofNiFe (18%) formed to a thickness between approximately 20 and 60angstroms with 40 angstroms being preferred. It is found that NiFe withapproximately 18% Fe by atom number used as the free layer givesparticularly good switching characteristics. A capping layer (130) isformed on the free layer.

As is understood by a person skilled in the art, the preferredembodiments of the present invention are illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods, materials, structures anddimensions employed in forming and providing an MTJ MRAM device in whichthe lower electrode has a smooth and flat layer structure and thenaturally oxidized tunneling barrier layer is thin, smooth and flat andhas a high breakdown voltage as a result of all layers being formed on asputter-etched Ta layer, while still forming and providing such a deviceand its method of formation in accord with the spirit and scope of thepresent invention as defined by the appended claims.

1. A magnetic tunneling junction (MTJ) device in an MRAM configurationcomprising: a substrate; a bottom magnetic keeper and conducting leadlayer formed on said substrate, said bottom keeper and lead layer formedof substantially planar layers and further comprising: a first NiCr seedlayer formed in said substrate; a soft magnetic keeper layer formed onsaid seed layer; a conducting lead layer formed on said keeper layer; aTa capping layer formed on said lead layer, the upper surface of saidlayer being rendered smooth and amorphous by being sputter-etched; abottom electrode, having smooth, flat layers, formed on saidsputter-etched Ta layer, said bottom electrode further comprising: asecond NiCr seed layer formed on said sputter-etched second Ta layer; apinning layer of antiferromagnetic material formed on said seed layer; aSyAP pinned layer formed on said pinning layer; a smooth, uniform,ultra-thin layer of in-situ radical oxidized (ROX) Al formed as abarrier layer on said pinned layer; a ferromagnetic free layer formed onsaid barrier layer; a capping layer formed on said MTJ layer.
 2. Thedevice of claim 1 wherein said substrate includes a planar insulatinglayer on which said bottom magnetic keeper and conductor lead layer isformed.
 3. The device of claim 1 wherein each of said first and secondseed layers is a layer of NiCr with 35–45 atom % Cr formed to athickness between approximately 40 and 60 angstroms.
 4. The device ofclaim 1 wherein the soft magnetic keeper layer is a layer of NiFe formedto a thickness between approximately 50 and 200 angstroms.
 5. The deviceof claim 1 wherein said conducting lead layer includes a third NiCr seedlayer with 35–45 atom percent Cr formed to a thickness betweenapproximately 40 and 60 angstroms on which is formed a Cu layer betweenapproximately 50 and 100 angstroms in thickness.
 6. The device of claim1 wherein said conducting lead layer includes a layer of Taapproximately 50 angstroms in thickness on which is formed a layer of Cubetween approximately 50 and 100 angstroms in thickness.
 7. The deviceof claim 1 wherein the Ta capping layer is formed to an originalthickness between approximately 50 and 100 angstroms and its surface issputter-etched, removing between approximately 20 and 30 angstroms oforiginal thickness and interrupting large grain formation and subsequentsurface roughness.
 8. The device of claim 1 wherein theantiferromagnetic pinning layer is a layer of MnPt formed to a thicknessof between approximately 100 and 200 angstroms or a layer of IrMn formedto a thickness between approximately 50 and 100 angstroms.
 9. The deviceof claim 1 wherein the SyAP layer comprises a first and second layer ofCoFe separated by a coupling layer of Ru, wherein at least the secondlayer of CoFe, which is adjacent to the tunnel barrier layer, isCoFe(25%).
 10. The device of claim 9 wherein the first layer of CoFe isapproximately 15 to 25 angstroms thick, the second layer of CoFe isapproximately 12 to 20 angstroms thick and the Ru layer is approximately7 to 8 angstroms thick.
 11. The device of claim 1 wherein the tunnelingbarrier layer is a layer of Al, formed to an initial thickness betweenapproximately 7 and 12 angstroms and oxidized in-situ by a process ofradical oxidation (ROX).
 12. The device of claim 1 wherein theferromagnetic free layer is a layer of NiFe(18%) formed to a thicknessbetween approximately 30 and 60 angstroms.